Permanent human habitation on a planetary body other than the Earth is one of science fiction's most prevalent themes. As technology has advanced, and concerns about the future of humanity on Earth have increased, the argument that space colonization is an achievable and worthwhile goal has gained momentum.[1][2] Other reasons for colonizing space include economic interests, long-term scientific research best carried out by humans as opposed to robotic probes, and sheer curiosity.

One of Elon Musk's stated goals through his company SpaceX is to make such colonization possible by providing transport, and to "help humanity establish a permanent, self-sustaining colony on [Mars] within the next 50 to 100 years".[3]

Earth is similar to Venus in bulk composition, size and surface gravity, but Mars's similarities to Earth are more compelling when considering colonization. These include:

The Martian day (or sol) is very close in duration to Earth's. A solar day on Mars is 24 hours, 39 minutes and 35.244 seconds.[4]

Mars has a surface area that is 28.4% of Earth's, only slightly less than the amount of dry land on Earth (which is 29.2% of Earth's surface). Mars has half the radius of Earth and only one-tenth the mass. This means that it has a smaller volume (~15%) and lower average density than Earth.

Mars has an axial tilt of 25.19°, similar to Earth's 23.44°. As a result, Mars has seasons much like Earth, though they last nearly twice as long because the Martian year is about 1.88 Earth years. The Martian north pole currently points at Cygnus, not Ursa Minor like Earth's.

Although there are some extremophile organisms that survive in hostile conditions on Earth, including simulations that approximate Mars, plants and animals generally cannot survive the ambient conditions present on the surface of Mars.[6]

Because Mars is approximately 52% farther from the Sun, the amount of solar energy entering its upper atmosphere per unit area (the solar constant) is only around 43.3% of what reaches the Earth's upper atmosphere.[14] However, due to the much thinner atmosphere, a higher fraction of the solar energy reaches the surface.[15][16][not in citation given] The maximum solar irradiance on Mars is about 590 W/m2 compared to about 1000 W/m2 at the Earth's surface. Also, year-round dust storms on Mars may block sunlight for weeks at a time.[17][18]

Mars's orbit is more eccentric than Earth's, increasing temperature and solar constant variations.

The Martian atmosphere is 95% carbon dioxide, 3% nitrogen, 1.6% argon, and traces of other gases including oxygen totaling less than 0.4%.

Martian air has a partial pressure of CO2 of 0.71 kPa, compared to 0.031 kPa on Earth. CO2 poisoning (hypercapnia) in humans begins at about 0.10 kPa. Even for plants, CO2 much above 0.15 kPa is toxic. This means Martian air is toxic to both plants and animals even at the reduced total pressure.[22]

Conditions on the surface of Mars are closer to the conditions on Earth in terms of temperature and atmospheric pressure than on any other planet or moon, except for the cloud tops of Venus.[23] However, the surface is not hospitable to humans or most known life forms due to greatly reduced air pressure, and an atmosphere with only 0.1% oxygen.

In 2012, it was reported that some lichen and cyanobacteria survived and showed remarkable adaptation capacity for photosynthesis after 34 days in simulatedMartian conditions in the Mars Simulation Laboratory (MSL) maintained by the German Aerospace Center (DLR).[24][25][26] Some scientists think that cyanobacteria could play a role in the development of self-sustainable manned outposts on Mars.[27] They propose that cyanobacteria could be used directly for various applications, including the production of food, fuel and oxygen, but also indirectly: products from their culture could support the growth of other organisms, opening the way to a wide range of life-support biological processes based on Martian resources.[27]

Humans have explored parts of Earth that match some conditions on Mars. Based on NASA rover data, temperatures on Mars (at low latitudes) are similar to those in Antarctica.[28] The atmospheric pressure at the highest altitudes reached by manned balloon ascents (35 km (114,000 feet) in 1961,[29] 38 km in 2012) is similar to that on the surface of Mars.[30]

Human survival on Mars would require complex life-support measures and living in artificial environments.

Mars presents a hostile environment for human habitation. Different technologies have been hypothesized to facilitate habitation on Mars, however many of them do not go into the psychological or biological effects involved in living on a foreign planet. Living in suits or in facilities creates an isolation effect that creates a deep depression. Scientists have also hypothesized that many different biological functions can be negatively affected by the environment of Mars colonies. Due to higher levels of radiation, there is a higher risk for cancer, especially if technological failures occur in facilities. The difference in gravity and the effects of artificial gravity could also negatively affect heart health and pregnancies, pertaining to "severe mental retardation [sic], head circumference, intelligence test scores, and school performance."[31] There are also concerns over the level of nutrition involved in early colonization periods. Degeneration is also a concern of scientists. Due to radiation and the different conditions on Mars, the concern comes with life expectancy and the viability of sustaining life. Sunlight is necessary for human life, and being able to have windows is affected by the radiation levels. Breakthroughs in engineering and technology may be able to fix these potential health risks. But early colonizing may be faced with these challenges and the harm could be seen for generations, as stated in academic articles: "the pioneers making the first journeys to Mars and its vicinity to explore and set up a base that eventually will lead to a continuously occupied colony, will face more hazards than those that follow."[32]

The psychological implications of space exploration and colonization follow mood disorders, such as depression and anxiety. However, these effects have been noted to be temporary. There aren’t many known cases of permanent psychological damage, namely because astronauts are screened before being cleared for missions. Some cases of psychological issues upon return to Earth have been noted, however. These cases have had to involve psychotherapy and mediation. The studies of psychological cases are based on near-orbit missions, and expeditions and colonization of Mars are hypothetical.[33]

A study from the Journal of Cosmology by Dr. Nick Kanas states that “Unprecedented factors will affect such a mission. A Mars crew will be tens of millions of miles away from home, engaged in a mission that will last around 21/2 years. Crew members [sic] will experience a severe sense of isolation and separation from the Earth, which will appear as a receding bluish-green dot in the heavens. From the surface of Mars, there will be 2-way communication delays with the Earth of up to 44 minutes, depending on where the two planets are located in their respective orbits, and the crew will be relatively autonomous from mission control.”[34] Current suggestions for Mars exploration and colonization are to select individuals who have passed psychological screenings. Psychosocial sessions for the return home are also suggested in order to reorient people to society.

There is much discussion regarding the possibility of terraforming Mars to allow a wide variety of life forms, including humans, to survive unaided on Mars's surface, including the technologies needed to do so.[35]

Mars has no global magnetosphere as Earth does. Combined with a thin atmosphere, this permits a significant amount of ionizing radiation to reach the Martian surface. The Mars Odyssey spacecraft carries an instrument, the Mars Radiation Environment Experiment (MARIE), to measure the radiation. MARIE found that radiation levels in orbit above Mars are 2.5 times higher than at the International Space Station. The average daily dose was about 220 µGy (22 mrad) – equivalent to 0.08 Gy per year.[36] A three-year exposure to such levels would be close to the safety limits currently adopted by NASA.[citation needed] Levels at the Martian surface would be somewhat lower and might vary significantly at different locations depending on altitude and local magnetic fields. Building living quarters underground (possibly in lava tubes that are already present) would significantly lower the colonists' exposure to radiation. Occasional solar proton events (SPEs) produce much higher doses.

Much remains to be learned about space radiation. In 2003, NASA's Lyndon B. Johnson Space Center opened a facility, the NASA Space Radiation Laboratory, at Brookhaven National Laboratory, that employs particle accelerators to simulate space radiation. The facility studies its effects on living organisms, as well as experimenting with shielding techniques.[37] Initially, there was some evidence that this kind of low level, chronic radiation is not quite as dangerous as once thought; and that radiation hormesis occurs.[38] However, results from a 2006 study indicated that protons from cosmic radiation may cause twice as much serious damage to DNA as previously estimated, exposing astronauts to greater risk of cancer and other diseases.[39] As a result of the higher radiation in the Martian environment, the summary report of the Review of U.S. Human Space Flight Plans Committee released in 2009 reported that "Mars is not an easy place to visit with existing technology and without a substantial investment of resources."[39] NASA is exploring a variety of alternative techniques and technologies such as deflector shields of plasma to protect astronauts and spacecraft from radiation.[39]

Mars requires less energy per unit mass (delta V) to reach from Earth than any planet except Venus. Using a Hohmann transfer orbit, a trip to Mars requires approximately nine months in space.[40] Modified transfer trajectories that cut the travel time down to four to seven months in space are possible with incrementally higher amounts of energy and fuel compared to a Hohmann transfer orbit, and are in standard use for robotic Mars missions. Shortening the travel time below about six months requires higher delta-v and an exponentially increasing amount of fuel, and is difficult with chemical rockets, and is also feasible with advanced spacecraft propulsion technologies, some of which have already been tested, such as Variable Specific Impulse Magnetoplasma Rocket,[41] and nuclear rockets. In the former case, a trip time of forty days could be attainable,[42] and in the latter, a trip time down to about two weeks.[43] In 2016, NASA scientists said they could further reduce travel time to Mars down to "as little as 72 hours" with the use of a "photonic propulsion" system instead of the fuel-based rocket propulsion system.[44]

During the journey the astronauts are subject to radiation, which requires a means to protect them. Cosmic radiation and solar wind cause DNA damage, which increases the risk of cancer significantly. The effect of long term travel in interplanetary space is unknown, but scientists estimate an added risk of between 1% and 19%, most likely 3.4%, for men to die of cancer because of the radiation during the journey to Mars and back to Earth. For women the probability is higher due to their larger glandular tissues.[45]

Mars has a gravity 0.38 times that of Earth and the density of its atmosphere is about 0.6% of that on Earth.[46] The relatively strong gravity and the presence of aerodynamic effects makes it difficult to land heavy, crewed spacecraft with thrusters only, as was done with the Apollo Moon landings, yet the atmosphere is too thin for aerodynamic effects to be of much help in aerobraking and landing a large vehicle. Landing piloted missions on Mars will require braking and landing systems different from anything used to land crewed spacecraft on the Moon or robotic missions on Mars.[47]

If one assumes carbon nanotube construction material will be available with a strength of 130 GPa then a space elevator could be built to land people and material on Mars.[48] A space elevator on Phobos has also been proposed.[49]

Colonization of Mars will require a wide variety of equipment—both equipment to directly provide services to humans and production equipment used to produce food, propellant, water, energy and breathable oxygen—in order to support human colonization efforts. Required equipment will include:[43]

Fuels or other energy source for use with surface transportation. Carbon monoxide/oxygen (CO/O2) engines have been suggested for early surface transportation use as both carbon monoxide and oxygen can be straightforwardly produced by zirconium dioxideelectrolysis from the Martian atmosphere without requiring use of any of the Martian water resources to obtain hydrogen.[51]

Communication equipment

3D Printers. These printers could be used to transform Martian soil and materials, along with materials brought from earth, into shelters and other land forms used by settlers. This process would not only require less supplies to be brought from Earth but also help with the efficient use and recyclability of materials.[52] Research is currently being done with simulated space materials to test the applicability of a 3D printer on Mars.[53]

According to Elon Musk, "even at a million people [working on Mars] you're assuming an incredible amount of productivity per person, because you would need to recreate the entire industrial base on Mars... You would need to mine and refine all of these different materials, in a much more difficult environment than Earth".[54]

Communications with Earth are relatively straightforward during the half-sol when Earth is above the Martian horizon. NASA and ESA included communications relay equipment in several of the Mars orbiters, so Mars already has communications satellites. While these will eventually wear out, additional orbiters with communication relay capability are likely to be launched before any colonization expeditions are mounted.

The one-way communication delay due to the speed of light ranges from about 3 minutes at closest approach (approximated by perihelion of Mars minus aphelion of Earth) to 22 minutes at the largest possible superior conjunction (approximated by aphelion of Mars plus aphelion of Earth). Real-time communication, such as telephone conversations or Internet Relay Chat, between Earth and Mars would be highly impractical due to the long time lags involved. NASA has found that direct communication can be blocked for about two weeks every synodic period, around the time of superior conjunction when the Sun is directly between Mars and Earth,[55] although the actual duration of the communications blackout varies from mission to mission depending on various factors—such as the amount of link margin designed into the communications system, and the minimum data rate that is acceptable from a mission standpoint. In reality most missions at Mars have had communications blackout periods of the order of a month.[56]

A satellite at the L4 or L5 Earth–Sun Lagrangian point could serve as a relay during this period to solve the problem; even a constellation of communications satellites would be a minor expense in the context of a full colonization program. However, the size and power of the equipment needed for these distances make the L4 and L5 locations unrealistic for relay stations, and the inherent stability of these regions, although beneficial in terms of station-keeping, also attracts dust and asteroids, which could pose a risk.[57] Despite that concern, the STEREO probes passed through the L4 and L5 regions without damage in late 2009.

Recent work by the University of Strathclyde's Advanced Space Concepts Laboratory, in collaboration with the European Space Agency, has suggested an alternative relay architecture based on highly non-Keplerian orbits. These are a special kind of orbit produced when continuous low-thrust propulsion, such as that produced from an ion engine or solar sail, modifies the natural trajectory of a spacecraft. Such an orbit would enable continuous communications during solar conjunction by allowing a relay spacecraft to "hover" above Mars, out of the orbital plane of the two planets.[58] Such a relay avoids the problems of satellites stationed at either L4 or L5 by being significantly closer to the surface of Mars while still maintaining continuous communication between the two planets.

The path to a human colony could be prepared by robotic systems such as the Mars Exploration RoversSpirit, Opportunity and Curiosity. These systems could help locate resources, such as ground water or ice, that would help a colony grow and thrive. The lifetimes of these systems would be measured in years and even decades, and as recent developments in commercial spaceflight have shown, it may be that these systems will involve private as well as government ownership. These robotic systems also have a reduced cost compared with early crewed operations, and have less political risk.

Wired systems might lay the groundwork for early crewed landings and bases, by producing various consumables including fuel, oxidizers, water, and construction materials. Establishing power, communications, shelter, heating, and manufacturing basics can begin with robotic systems, if only as a prelude to crewed operations.

Before any people are transported to Mars on the notional 2030s Interplanetary Transport System envisioned by SpaceX, a number of robotic cargo missions would be undertaken first in order to transport the requisite equipment, habitats and supplies.[61] Equipment that would be necessary would include "machines to produce fertilizer, methane and oxygen from Mars' atmospheric nitrogen and carbon dioxide and the planet's subsurface water ice" as well as construction materials to build transparent domes for initial agricultural areas.[62]

In 1948, Wernher von Braun described in his book The Mars Project that a fleet of 10 spaceships could be built using 1000 three-stage rockets. These could bring a population of 70 people to Mars.

All of the early human mission concepts to Mars as conceived by nationalgovernmental space programs—such as those being tentatively planned by NASA, FKA and ESA—would not be direct precursors to colonization. They are intended solely as exploration missions, as the Apollo missions to the Moon were not planned to be sites of a permanent base.

Colonization requires the establishment of permanent bases that have potential for self-expansion. A famous proposal for building such bases is the Mars Direct and the Semi-Direct plans, advocated by Robert Zubrin.[43]

As with early colonies in the New World, economics would be a crucial aspect to a colony's success. The reduced gravity well of Mars and its position in the Solar System may facilitate Mars–Earth trade and may provide an economic rationale for continued settlement of the planet. Given its size and resources, this might eventually be a place to grow food and produce equipment to mine the asteroid belt.

A major economic problem is the enormous up-front investment required to establish the colony and perhaps also terraform the planet.

Some early Mars colonies might specialize in developing local resources for Martian consumption, such as water and/or ice. Local resources can also be used in infrastructure construction.[66] One source of Martian ore currently known to be available is metallic iron in the form of nickel–iron meteorites. Iron in this form is more easily extracted than from the iron oxides that cover the planet.

Another main inter-Martian trade good during early colonization could be manure.[67] Assuming that life doesn't exist on Mars, the soil is going to be very poor for growing plants, so manure and other fertilizers will be valued highly in any Martian civilization until the planet changes enough chemically to support growing vegetation on its own.

Solar power is a candidate for power for a Martian colony. Solar insolation (the amount of solar radiation that reaches Mars) is about 42% of that on Earth, since Mars is about 52% farther from the Sun and insolation falls off as the square of distance. But the thin atmosphere would allow almost all of that energy to reach the surface as compared to Earth, where the atmosphere absorbs roughly a quarter of the solar radiation. Sunlight on the surface of Mars would be much like a moderately cloudy day on Earth.[68]

Space colonization on Mars can roughly be said to be possible when the necessary methods of space colonization become cheap enough (such as space access by cheaper launch systems) to meet the cumulative funds that have been gathered for the purpose.

Although there are no immediate prospects for the large amounts of money required for any space colonization to be available given traditional launch costs,[69][full citation needed] there is some prospect of a radical reduction to launch costs in the 2010s, which would consequently lessen the cost of any efforts in that direction. With a published price of US$56.5 million per launch of up to 13,150 kg (28,990 lb) payload[70] to low Earth orbit, SpaceXFalcon 9 rockets are already the "cheapest in the industry".[71] SpaceX's reusable plans include Falcon Heavy and future methane-based launch vehicles including the Interplanetary Transport System. If SpaceX is successful in developing the reusable technology, it would be expected to "have a major impact on the cost of access to space", and change the increasingly competitive market in space launch services.[72]

Mars Odyssey found what appear to be natural caves near the volcano Arsia Mons. It has been speculated that settlers could benefit from the shelter that these or similar structures could provide from radiation and micrometeoroids. Geothermal energy is also suspected in the equatorial regions.[74]

Lava tubes

Several lava tube skylights on Mars have been located on the flanks of Arsia Mons. Earth based examples indicate that some should have lengthy passages offering complete protection from radiation and be relatively easy to seal using on-site materials, especially in small subsections.[75]

Robotic spacecraft to Mars are required to be sterilized, to have at most 300,000 spores on the exterior of the craft—and more thoroughly sterilized if they contact "special regions" containing water,[76][77] otherwise there is a risk of contaminating not only the life-detection experiments but possibly the planet itself.

It is impossible to sterilize human missions to this level, as humans are host to typically a hundred trillion microorganisms of thousands of species of the human microbiome, and these cannot be removed while preserving the life of the human. Containment seems the only option, but it is a major challenge in the event of a hard landing.[78] There have been several planetary workshops on this issue, but with no final guidelines for a way forward yet.[79] Human explorers would also be vulnerable to back contamination to Earth if they become carriers of microorganisms.[80]

It is unforeseen how the first human landing on Mars will change the current policies regarding the exploration of space and occupancy of celestial bodies. In the 1967, United Nations Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies, it was determined that no country may take claim to space or its inhabitants. Since the planet Mars offers a challenging environment and dangerous obstacles for humans to overcome, the laws and culture on the planet will most likely be very different from those on Earth.[81] With Elon Musk announcing his plans for travel to Mars, it is uncertain how to the dynamic of a private company possibly being the first to put a human on Mars will play out on a national and global scale.[82][83] With NASA having recently taken a cut in funding, it is no longer certain that they will be a leading force in the journey to Mars.[84] It is likely that the values and culture on Mars will be determined by the group that establishes human contact with the planet first.

One possible ethical challenge that space travelers might face is that of pregnancy during the trip. According to NASA’s policies, it is forbidden for members of the crew to engage in sex in space. NASA wants its crewmembers to treat eachother like coworkers would in a professional environment. A pregnant member on a spacecraft is dangerous to all those aboard. The pregnant woman and child would most likely need additional nutrition from the rations aboard, as well as special treatment and care. At some point during the trip, the pregnancy would most likely impede on that crew member's duties and abilities. It is still not fully known how the environment in a spacecraft will affect the development of a child aboard. It is known however that an unborn child in space would be more susceptible to solar radiation, which would likely have a negative effect on its cells and genetics.[85] During a long trip to Mars it is likely that members of craft may engage in sex due to their stressful and isolated environment. It is possible that prior to the departure of a colonization mission, crew members may be required to be sterilized. If this is the chosen method, will all members be required to undergo this process or just one gender of the crew?[86]

Mars colonization is advocated by several non-governmental groups for a range of reasons and with varied proposals. One of the oldest groups is the Mars Society who promote a NASA program to accomplish human exploration of Mars and have set up Mars analog research stations in Canada and the United States. Mars to Stay advocates recycling emergency return vehicles into permanent settlements as soon as initial explorers determine permanent habitation is possible. Mars One, which went public in June 2012, aims to establish a fully operational permanent human colony on Mars by 2027 with funding coming from a reality TV show and other commercial exploitation, although this approach has been widely criticized as unrealistic and infeasible.[87][88][89]

Elon Musk founded SpaceX with the long-term goal of developing the technologies that will enable a self-sustaining human colony on Mars.[82][90] In 2015 he stated "I think we've got a decent shot of sending a person to Mars in 11 or 12 years".[91]Richard Branson, in his lifetime, is "determined to be a part of starting a population on Mars. I think it is absolutely realistic. It will happen... I think over the next 20 years, we will take literally hundreds of thousands of people to space and that will give us the financial resources to do even bigger things".[92]

^Queens University Belfast scientist helps NASA Mars project "No-one has yet proved that there is deep groundwater on Mars, but it is plausible as there is certainly surface ice and atmospheric water vapour, so we wouldn't want to contaminate it and make it unusable by the introduction of micro-organisms."

^Safe on Mars page 37 "Martian biological contamination may occur if astronauts breathe contaminated dust or if they contact material that is introduced into their habitat. If an astronaut becomes contaminated or infected, it is conceivable that he or she could transmit Martian biological entities or even disease to fellow astronauts, or introduce such entities into the biosphere upon returning to Earth. A contaminated vehicle or item of equipment returned to Earth could also be a source of contamination."